The Bible assures us that God will forgive every sin we have ever committed, if we turn to Christ in repentance and faith, and trust our lives into His hands. The Bible says that God "forgives all your sins" (Psalm 103:3).
Most of us, I suspect, can look back and recall times when we weren't as grateful as we should have been for what someone did for us. Why were we like this? The reason is because all too often we think only of ourselves, forgetting all about the other person and the sacrifices they made for us. In other words, we become selfish and insensitive -- and that a sin. People, the Bible says, then become "lovers of themselves... ungrateful, unholy, without love" (2 Timothy 3:2-3).
But God doesn't want us to be like this -- and that's why He not only wants to forgive us, but to change our hearts and make us more like Christ. And this is possible as we turn to Christ for the forgiveness and new life we need. By a simple prayer of faith ask Him to come into your life today. Jesus' promise is sure: God "is kind to the ungrateful and wicked" (Luke 6:35).
No, you didn't thank your grandmother as you should have -- and I hope you'll learn from this experience to be more grateful to others, even in small things. But you still can thank God for giving her to you, and for helping her even in hard times to serve Him, by serving you.